The present invention relates to a swing-type actuator such as an actuator for magnetic disk drives, and more particularly to a swing-type actuator capable of swinging a function member such as a magnetic head along a circular course.
Conventionally, the positioning of a magnetic head on a recording track of a magnetic disk, etc. is conducted by a swing-type or rotation-type actuator as shown in FIGS. 11 and 12. In both figures, a yoke 1 is fixedly provided with permanent magnets 2, and a pair of yokes 1 are assembled by supports 3 such that different magnetic poles of the permanent magnets 2 are opposite each other via a magnetic gap 4 to form a magnetic circuit. 5 represents an arm having one end to which a flat movable coil 6 is fixed, and the other end to which a magnetic head (not shown) is fixed. The arm 5 is arranged such that the movable coil 6 located in the magnetic gap 4 can swing around a shaft 7.
When an operation signal is supplied to the movable coil 6, a magnetic force generated from the movable coil 6 according to Fleming's left hand rule functions as an attraction force or a repulsion force to each permanent magnet 2, so that the arm 5 is rotated around the shaft 7. As a result, a magnetic head fixed to a tip end of the arm 5 is positioned on a desired magnetic track of a magnetic disk (not shown). The direction of the rotation of the arm 5 can be changed by inverting the direction of current applied to the movable coil 6. To regulate the swing span of the movable coil 6, there may be stoppers (not shown) with which inner surfaces of the movable coil 6 are brought into contact.
In the above conventional actuator for magnetic disk drives, the movable coil 6 is usually fixed to the arm 5 by an adhesive. However, the fixing of the movable coil 6 by an adhesive is sometimes troublesome, failing to provide accurate positioning of the movable coil 6. In addition, handling of terminals of the movable coil 6 is complicated, lowering the efficiency of assembling of the arm 5. Since there is increasingly higher demand for miniaturization and reduction in thickness of magnetic disk drives, it is necessary to improve the positioning accuracy of the movable coil 6, and the efficiency and reliability of fixing of the movable coil 6 to the actuator 5. In this sense, the conventional arms fail to satisfy these requirements.
To solve the above problems, attempts have been made to integrally fix a movable coil 6 to an arm 5 by an integral resin molding (for instance, U.S. Pat. No. 4,855,853 and Japanese Utility Model Laid-Open No. 60-159566). In such a structure, the movable coil can be supported by a simplified structure, and the thickness of the movable coil can be extremely reduced. Accordingly, such a structure is advantageous for miniaturizing actuators. Also, miniaturization can be achieved by fixing one or more permanent magnets 2 to only one of the yokes 1, 1 to constitute the magnetic circuit.
However, in the above resin-molded structure, there are several problems.
First, the resin molding structure is generally produced by an injection molding method, but the resulting resin molding is likely to have a flash. Specifically, in the injection molding, the arm 5 and the movable coil 6 having lead wires to which terminal pins (not shown) are soldered are placed in a cavity of an injection mold, and a molten thermoplastic resin is injected into the cavity. After solidification, the resulting molding is taken out of the mold. By injection molding, the arm 5 and the movable coil 6 are integrally molded.
The injection mold is constituted by a pair of mold halves which are divided along a parting line. A parting face of each mold half is worked at a high precision, and there is desirably no gap between the mold halves along the parting line when they are closed. However, by use for a long period of time, the mold faces are worn, resulting in a slight gap along the parting line depending upon a mold compression force and an injection pressure. Due to such a gap, the resin injected into a cavity of the mold enters into the gap, resulting in the formation of a flash.
Also, when the arm has a cylindrical outer surface, there is inevitably a clearance between the arm and an inner surface of the cavity, because of a dimensional error of the arm, etc. Accordingly, there is also a gap. When the resin enters into the gap, a foil-shaped or film-shaped flash is formed in a boundary between the arm and the resin molding part.
The existence of the flash deteriorates the appearance of the actuator, and if the flash falls onto a recording medium such as a magnetic disk during the operation, it interferes with the reading and writing function of the reading medium, lowering the reliability of a recording apparatus.
It is generally extremely difficult to make the mold halves free from a gap along a parting line, and if there is no gap, the mold halves would be rather rapidly worn, and an injection molding operation would become less efficient. Accordingly, at present the flash is removed from the resin molding part of the finished arm manually. This manual removal of the flash is a labor-intensive, lengthy operation.
Second, it is possible to achieve the reduction of weight by using a structure in which a periphery of the movable coil 6 is held by molding. However, when the movable coil 6 is thin as in a 2.5-inch-FDD, a good balance with the arm 5 cannot be maintained. Particularly, in the case of a swing-type actuator having many kinds of function members attached to the arm 5, the operation of the arm 5 cannot be conducted smoothly. Also, in a case where the movable coil 6 is thin, the movable coil 6 does not have a sufficient rigidity.
Third, in the above injection molding, terminal pins to which lead wires of the movable coil are connected are usually provided on or near a line connecting a center of the arm and a center of the movable coil, and to achieve easy connection with an external power supply, it is advantageous that the terminal pins are provided near the side edge of the movable coil. However, in such a structure, a pair of lead wires to be connected with the terminal pins are inevitably disposed close to each other.
Accordingly, this structure is disadvantageous in that good insulation is not always ensured between the lead wires and the movable coil, because it is likely that the lead wires are brought into contact with each other or with wrong terminal pins due to the resin flow during the injection molding. Also, depending upon the direction of the resin flow, the lead wires may be exposed from a surface of the resin molding or may be cut in an extreme case.